For the direct channel component from each user to the antenna array of the BS, the far-field condition is practically satisfied due to the much longer signal propagation distance between the user and BS w.r.t. the size of the BS's UPA. Thus, the UPW model can be used to characterize the channel response, where the one-dimensional (1D) array response vector can be defined as [1]}
\(\mbox{{$ e $}}(\phi ,\bar{M})=[1,e^{i\pi \phi },\cdots ,e^{i(\bar{M}-1)\pi \phi }]^{T},\) 
